UPDATE: NASA's MESSENGER Enters Mercury Orbit

The Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft has become the first to orbit the solar system's innermost planet. Its findings could unlock the secrets of how Mercury formed, and help scientists to understand what our solar system was like at its very beginning.

UPDATE (9:39 p.m., March 17): Success! Just after 9 p.m. EDT last night, March 17, the MESSENGER spacecraft completed its 17 minute-long process of entering orbit of Mercury. The spacecraft will now spend the next year in elliptical orbit. Mission scientists plan to begin collecting data on April 4, once they activate all seven of MESSENGER's instruments.

Tonight, if all goes well, NASA's MESSENGER probe will complete a seven-year journey by entering orbit of Mercury. Launched in 2004, the probe–whose full name is the Mercury Surface, Space Environment, Geochemistry, and Ranging spacecraft—will be the first in history to orbit the closest planet to the Sun, and its findings will help scientists answer fundamental questions about the dawn of the solar system.

Why Mercury? Though humans have known about the planet since ancient times, it remains shrouded in mystery, according to Ralph McNutt, project manager of MESSENGER's core team. Scientists have been puzzled about how Mercury formed, because Mercury features a metal-rich core that accounts for about 60 percent of the planet's mass, twice that of any of the other terrestrial planets—Venus, Earth or Mars.

For the past 30 years, there have been three main ideas about the formation of Mercury, according to MESSENGER principal investigator Sean C. Solomon. One idea holds that the primordial soup of our solar system was not uniform, and that a high concentration of metals was rotating close to the sun, causing Mercury's composition to be primarily metal. A second hypothesis proposes that Mercury formed very quickly while still surrounded by superheated gas, which vaporized most of its rocky outer crust. A third idea says that young Mercury was in a huge collision. "If Mercury was hit by something almost its own size, in the right way, it could have ejected much of the outer part of the planet," Solomon says.

To test these hypotheses, MESSENGER will carry an array of spectrometers to study the elemental composition of the planet's surface. "That's going to go a long way toward constraining some of the ideas," McNutt says. Each of the three ideas has a particular signature of elements left behind on Mercury that would support it—for instance, if MESSENGER spots more calcium, aluminum, and silicon than iron, that would support the second idea: superheated gas. The probe also carries a complex digital camera called the Mercury Dual Imaging System, which features a wide angle, multi-spectral camera for observing the different types of rock on the planet and a narrow-angle camera that will capture 98 percent of the planet's surface as MESSENGER completes its 12-hour orbits. Before MESSENGER, the last craft to visit Mercury was Mariner 10, which photographed just 45 percent of the surface during flybys in 1974 and 1975.

NASA waited so long to return to Mercury—this time with an orbiter—because of two major challenges. "The biggest one, which we didn't know the solution to in 1975, was how to slow it down enough," Solomon says. A craft launched from Earth would be traveling more than 10 kilometers per second upon reaching the innermost planet, he says, but it would need to slow down to less than 0.09 kilometers per second to achieve successful orbit. Back in Mariner 10's time, Solomon says, "there was no propulsion system that our spacecraft could carry that could slow the spacecraft." The second hurdle was cost. In 1985, Chen Wan Yin, a NASA mission design expert, came up with a way to send an orbiter to Mercury via multiple flybys of Venus and Mercury itself, using their gravity to slow the spacecraft. But a 1992 proposal to send twin probes was estimated to cost $1 billion, McNutt says, "which is why the idea was dead on arrival." The $280 million MESSENGER mission—which costs so much less than the previous proposal partly because it didn't need to be launched via space shuttle mission—finally got the green light in 1999. The program then had to wait for the planets to align to create a launch window in 2004.

If MESSENGER succeeds in entering orbit, the spacecraft will have to endure both heat from the sun, which is 11 times greater than what we receive here on Earth, and heat radiating off Mercury's surface, which Solomon says can get up to 350 degrees Celsius. The team developed a sunshade made of ceramic cloth like that found on the space shuttles, which surrounds layers of thermal-resistant Kapton plastic. The shade faces the Sun at all times, keeping the rest of the orbiter at room temperature.

A live webcast of the MESSENGER probe orbital insertion will be broadcast from the mission website starting at 7:55 p.m. EDT tonight, and MESSENGER should begin entering orbit at 8:45 p.m. EDT

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